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Efficacy of Low-Carb Ketogenic Diet in Endurance Athletes

Improvements demonstrated in body composition and athletic performance

by Bianca Garilli, ND, IFMCP

Endurance-trained athletes must maintain excellent physical fitness in order to regularly perform activities requiring large quantities of sustained energy. Nutrition plays a major role in their ability to achieve superior athletic goals. However, athletes often do not meet recommended nutritional requirements based on sport, age and sex.1

A study conducted in 146 endurance athletes from Lithuania found that 80.8% of endurance athletes used lower than recommended amounts of carbohydrates in their diet, while more than 70% of the athletes consumed higher than recommended levels of total fat, saturated fat, and cholesterol.1 Female athletes demonstrated the largest nutrient gaps, with low intakes of carbohydrates, dietary fiber, protein, omega-3 fatty acids, B vitamins, potassium, calcium, phosphorus, iron, manganese, and zinc.1 Age also played a role, with adolescent athletes aged 14-18 years frequently consuming either excessive or inadequate protein for their needs.1

The importance of optimizing and individualizing the diets of highly trained endurance athletes requires constant exploration and analysis of novel approaches to nutrition. A publication in Metabolism studied 20 male endurance-trained athletes who had previously been following a carbohydrate-based diet.2 The objective of the study was to determine if altering their diet to a low-carbohydrate, high-fat ketogenic diet (LCKD) would improve body composition and performance outcomes in these well-trained athletes.2

Participants were separated into two groups; the first group remained on a high-carbohydrate (HC) diet (C:P:F* = 65:15:20), and the second group consumed a LCKD (C:P:F = 6:17:77).2 Both groups remained on the specified diet for 12-weeks.2 During this time, both groups followed an athletic training program including endurance, strength and high intensity interval training (HIIT).2 Prior to the start of the study and at the conclusion of the study, both group’s body composition was assessed and participants performed a 100 km time trial (TT), a six second (SS) sprint, and a critical power test (CPT).2


  • The LCKD group experienced:
    • Significantly greater decrease in body mass and body fat % when compared to the HC group
    • Significantly higher fasting serum beta-hydroxybutyrate (ketone body) when comparing baseline to week 12 measurements
  • SS sprint peak power: increased by 0.8 watts per kilogram bodyweight (w/kg) in the LCKD group and decreased by 0.1 w/kg in the HC group
  • CPT peak power: increased by 1.4 w/kg in the LCKD group and decreased by 0.7 w/kg in the HC group
  • The LCKD group experienced a significantly greater fat oxidation level throughout the 100 km TT

While improvements were noted in the LCKD intervention arm,2 further research is warranted to understand the efficacy of the ketogenic diet in different types of athletic performance. A randomized crossover trial demonstrated that the ketogenic diet impaired anaerobic exercise performance in exercise-trained men and women.3

Why is this Clinically Relevant?

  • Many endurance athletes are not meeting nutritional recommendations to perform at their peak capacities
  • LCKD may improve peak performance in those athletes needing to optimize sprint speed and peak power
  • LCKD may support greater fat oxidation during athletic performance
  • LCKD lowers body mass and body fat percentage
  • Nutritional recommendations should be individualized for sport, age and sex

* C:P:F = indicates the carbohydrate to protein to fat ratio

Link to abstract


  1. Baranauskas M, Stukas R, Tubelis L, et al. Nutritional habits among high-performance endurance athletes. Medicina (Kaunas). 2015;51(6):351-62.
  2. McSwiney FT, Wardrop B, Hyde PN, Lafountain RA, Volek JS, Doyle L. Keto-adaptation enhances exercise performance and body composition responses to training in endurance athletes. Metabolism. 2018;81:25-34.
  3. Wroble KA, Trott MN, Schweitzer GG, Rahman RS, Kelly PV, Weiss EP. Low-carbohydrate, ketogenic diet impairs anaerobic exercise performance in exercise-trained women and men: a randomized-sequence crossover trial. J Sports Med Phys Fitness. 2018;59(4):600-607.

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